January 08, 2008

Carbon nanotubes on plastic 312 megahertz instead of kilohertz for current plastic circuits

Scientists from the University of Massachusetts Lowell and Brewer Science, Inc. have used carbon nanotubes as the basis for a high-speed (312 megahertz) thin-film transistors printed onto sheets of flexible plastic. Their method may allow large-area electronic circuits to be printed onto almost any flexible substrate at low cost and in mass quantities. (Most Intel and AMD processors are in the 2 to 3 Gigahertz range)

The Pentium II (mid-97 to 1999) had processor speeds of 233-450 Mhz The new carbon nanotubes of plastic have processor speeds in that range and can be printed in large sheets in process taht is similar to inkjet printing.

Applications for these flexible electronics include electronic paper, RFID (radio frequency identification) tags to track goods and people, and “smart skins,” which are materials and coatings containing electronic circuitry that can indicate changes in temperature or pressure, such as on aircraft or other objects.

Our electronic-grade solutions contain ultrapure carbon nanotubes without using any surfactant. Our printed transistor's carrier mobility is much higher than similar devices developed by other groups, it exhibits a speed of 312 megahertz, and can carry a large current.

As part of the printed-electronics effort, carbon nanotubes have been investigated as a medium for high-speed transistors, with very promising results. But one method of depositing the nanotubes onto the plastic, “growing” them with heat, requires very high temperatures, typically around 900°C, which is a major obstacle for fabricating electronic devices.

Brewer Science, Inc. developed an electronic-grade carbon-nanotube solution. The researchers deposited a tiny droplet of the solution onto a plastic transparency film at room temperature using a syringe, a method similar to ink-jet printing.

Other printable electronics.

Kovio's inkjet printable electronics has electron mobility of ~80 cm2/(V·s).

Many companies and R&D labs have been aiming at getting the electron mobility—expressed in units of cm2/(V·s)—of organics semiconductors up to the 0.5-1.0 range of amorphous-silicon TFTs.

Previous reports have shown that C60 can yield mobility values as high as six square centimeters per volt-second (6 cm2/V/s). However, that record was achieved using a hot-wall epitaxy process requiring processing temperatures of 250 degrees Celsius – too hot for most flexible plastic substrates.

Though the transistors produced by Kippelen’s research team display slightly lower electron mobility – 2.7 to 5 cm2/V/s – they can be produced at room temperature.

Graphene has high potential. Electron mobility in graphene is 200,000 cm2/Vs and more than 100 times higher than for silicon - researchers believe graphene has the potential to improve upon the capabilities of current semiconductors and open up exciting new possibilities. These include ultra-high frequency detectors required for full-body security scanners, which would make people transparent by operating at terahertz (THz) frequencies. However, most estimate it will be 20 years before that full potential will be realized.

Fuhrer's group measured 100,000-cm2/volt-second mobility at room temperature for the nanotube structures. That's about 70 times the 1,500-cm2/V-s mobility of standard silicon chips and 10 times the 10,000 cm2/V-s achievable by silicon's mobility leaders, discrete high-electron-mobility transistors (HEMTs). The mobility record set by InSb in 1955 was 77,000.